Alexandrite, a rare variety of chrysoberyl, is prized for its remarkable optical properties. In daylight, which is rich in shorter wavelengths, it looks bright green. In the warmer, longer-wavelength light of candles or tungsten bulbs, it takes on a rich red or brownish-red colour. It has therefore been described as ‘EMERALD by day, RUBY by night’. The colour change is due to chromic oxide, which in alexandrite partially replaces the aluminium oxide that occurs in the chemical composition of chrysoberyl. Colour changes occur in some types of SAPPHIRES, TOURMALINE, APATITE, and many other gemstones, but alexandrite shows the most dramatic effect.

An important source of alexandrite was the Ural Mountains of Russia, where the variety was first described in 1830. The story has it that it was discovered by emerald miners on the birthday of the future Tsar Alexander II, and was named in his honour by the mineralogist Nils Nordenskjold. The name was doubly appropriate since red and green were the Russian imperial colours. The gemstone became popular in Russian jewellery and the Urals mines were soon exhausted. In the 1920s, Tiffany produced some beautiful rings with alexandrite set in platinum.

For much of the 20th century, with no new major discoveries of the mineral, alexandrite was extremely rare. Then in 1987, a new find was made at Hematita, Minas Gerais, Brazil. Alexandrite from Hematita has a dramatic colour change from raspberry red to bluish green. In 1993, there was another major find of alexandrite on the border of Tanzania and Mozambique.